1. Field of the Invention
This invention relates to a pressure transducer.
More particularly, the invention relates to a pressure transducer of a type comprising a housing provided with a cylindrical passage therein and a pressure connection at one end of the passage, a substantially cylindrical member arranged in said passage so as to be longitudinally slidable therein, with sealing means between said member and the passage, so that pressure applied at said one end of the passage via said pressure connection biases said member away therefrom, a spring for biasing said member towards said one end against said pressure, and rack and pinion means within the housing for converting linear displacement of said member into rotary displacement of the pinion, the rack being formed by grooves in the member itself.
2. Description of the Prior Art
French Patent Specification No. 506 915 discloses such a type of presssure transducer in FIGS. 1 to 3 thereof. However, the pressure gauge of FIGS. 1 to 3 of FR-PS No. 506 915 is not a differential pressure gauge. Furthermore, it necessarily uses positive sliding seals. Furthermore, the rack of the rack and pinion means is formed by a tubular body c2 which is not itself a pressure-sensing element but is attached to a positively sealed piston c which depends upon a flexible lipped seal c1. The relatively high level of frictional hysteresis generated by such sealing means is well known.
FIG. 5. of French Patent Specification No. 506 915 relates to a simple pressure gauge (not a differential pressure gauge) utilising a piston member with opposed differential areas so as to reduce the force to be opposed by the spring at high pressures. In this case there are two sliding flexible seals, each resisting and fully containing the full system pressure relative to atmosphere. The sensitivity of such an instrument would of course be very limited.
Other, less relevant prior art, includes GB No. 2 010 596 A, which discloses an example of a magnetic coupling which uses a non-magnetic "window" in an otherwise magnetic casing; DE No. 2 747 047, which discloses another and very elaborate, form of magnetic coupling; U.S. Pat. No. 4,014,284, which discloses a relatively crude device, not intended for accurate measurement of differential pressure, because of a great deal of friction produced at the flat seating faces of both pairs of magnets as a consequence of their respective mutual attractions; it is not a gauge as such, but an indicator in which friction can be allowed for because the device need only react to increasing pressure; the mechanism for converting linear to angular movement is functionally separate from the piston (although connected to it); and finally U.S. Pat. No. 3,429,291 and FR No. 2 470 374, neither of which is particularly relevant because each depends on purely magnetic coupling means with one magnet actually incorporated into the piston itself.